Traumatic injury is the third leading cause of death in the US and about 40% of these deaths are due to hemorrhage. One quarter of all trauma patients experience prolonged hemorrhage due to impaired blood clotting (coagulopathy). These patients suffer from excessive bleeding disproportionate to their injuries, a phenomenon that is poorly understood. Achieving hemostasis following injury is crucial to controlling blood loss and platelets are critical regulators of this process. Platelet adhesion to damaged endothelium is mediated by von Willebrand factor (vWF) binding to the GPIb? receptor on activated platelets. Preliminary data show that platelet adhesion to vWF was significantly reduced following injury. In addition, these patients displayed reduced platelet GPIb? receptor expression and increased shedding of GPIb? into the plasma, suggesting a possible mechanism behind defective platelet-vWF adhesion. The long-term goal is to understand the contribution of platelet receptor shedding to platelet dysfunction and bleeding in trauma patients. Shedding of GPIb? is proteolytically regulated by tumor necrosis factor-? converting enzyme (TACE), which modulates platelet-vWF adhesion through cleavage of GPIb?. Preliminary clinical results show a substantial increase in TACE expression on platelets following injury. Therefore, the objective of this application is to determine the contribution of TACE-mediated GPIb? receptor shedding to the defective platelet-vWF adhesion observed following traumatic injury. The central hypotheses are: 1) Excessive hemorrhage in trauma patients results from defective platelet-vWF adhesion due to inappropriate shedding of GPIb?; 2) This shedding is mediated by elevated TACE, and can be corrected through TACE inhibition. The rationale for this research is that understanding the effects of injury on platelet-vWF binding will allow identification of the mechanisms behind life-threatening hemorrhage and develop pharmacologic interventions to alleviate bleeding. These hypotheses will be tested by pursuing two specific aims: 1) Identify the mechanisms behind defective platelet-vWF adhesion following traumatic injury; and 2) Characterize the effect of TACE absence and inhibition on platelet GPIb? receptor expression and hemostasis in a mouse model of trauma and hemorrhage. Under the first aim, serial blood samples will be collected from severely injured patients to measure effects of injury and resuscitation fluids on expression of platelet GPIb? and TACE by flow cytometry and shedding of GPIb? by ELISA. Under the second aim, TACE absence and inhibition will be tested in a mouse model using both platelet-specific TACE knock-out mice and TACE inhibitors in wild-type mice to determine the effects of TACE absence and inhibition on preserving GPIb? receptor expression. This approach is innovative because it utilizes both human and animal subjects and a unique patient population rarely studied in the lab setting. The proposed research is significant because the results will identify strategies to facilitate more effective platelet- vWF binding, which could subsequently improve primary hemostasis in hemorrhaging trauma patients.